Gravity Assists
Gravity assists are performed to save fuel at the expense of time. They have been used by many spaceships throughout history. Real Ships Without gravity assists, the well known Voyager Grand Tour would never have been possible. Jupiter gravity assists helped Cassini reach Saturn. However, they have been largely used for smaller space missions. For example, ESA's Rosetta probe used two Earth and one Mars gravity assist to reach a comet and NASA's Messenger probe used many Earth, Venus and Mercury flybys to enter Mercury orbit. Simulator Gravity assists are largely used during a Mercury Return Mission and usually use Venus. However, they can be performed with all other planets and the Moon. Mercury During a Mercury mission, the ship might reach Mercury's sphere of influence, but too fast. In order to decrease speed, one or two Mercury flybys should be needed. In addition, when a ship leaves Mercury, a flyby can increase its aphelion to the orbit of Venus. Mercury has a very low sphere of influence and little mass. Because of this, gravity assists are difficult to perform. Venus Venus is largely used for gravity assists. The planet has enough mass. with one or two gravity assists, a ship coming from Earth can be sent to Mercury and a ship coming from Mercury can be sent to Earth. In addition, Venus is used for ships that have only little fuel left, that cannot reach the Earth in one flight. A Mercury Return Mission or a Venus Return Mission might use all or nearly all fuel of a spaceship. However, if the astronaut waits for a correct planetary alignment and uses a Venus flyby, it can rise the aphelion of its ship to reach Earth orbit. Earth The Earth can be used for a ship passing from Venus to Mars or from Mars to Venus. However, this is something done rarely. More often, an Earth gravity assist can be used for a ship in heliocentric orbit, to get to Venus or Mars, saving fuel. NASA sent the probe Osiris Rex to have an Earth gravity assist, in order to reach an asteroid. The ship Deep Impact performed several Earth flybys to gently adjust its orbit, to encounter a comet. Warning! The Moon might sometimes get in the way of an Earth gravity assist. Moon The Moon has little mass, but still can be used for a gravity assist. One or two Moon flybys can send a ship into heliocentric orbit. NASA's Stereo probes used a Moon flyby for this reason. Mars Mars was used for Dawn and Rosetta missions for a gravity assist. A Mars flyby can send a ship into the Asteroid Belt. In addition, a Mars flyby can lower the perihelion closer to Earth. Phobos & Deimos The two small moons of Mars have very little mass. A flyby can change the orbit of a ship by very little. Even if in theory multiple flybys could send a ship out of the Martian sphere of influence, in practice this is not feasible. The Sun In some simulators, like the Kerbal Space Program, it is possible to use a Sun gravity assist, a thing that in reality is not possible (except for an interstellar mission, for a ship approaching from another star). In Spaceflight Simulator, this is not possible. Technique To perform a gravity assist, there are a few things that must be considered: * A flyby with a planet close to the perihelion will usually lower the perihelion. ** To get closer to the Sun, enter the sphere of influence of the target planet from between the planet and the Sun. * A flyby with a planet close to the aphelion will usually rise your aphelion. ** To get away from the Sun, enter the sphere of influence of the target planet from a point away from the Sun. * The closer you get to the planet, the stronger will be the effect. Beware not to hit the surface or enter the atmosphere. **At closest approach, you can fire the engines forwards to increase the effect and backwards to decrease the effect. You will take advantage of the Oberth effect and save fuel. Map View When the path of your ship will intersect the sphere of influence of a planet, you can see on map view that the trajectory ends. If you zoom in to the planet, you can see the path of your ship inside the sphere of influence. The path you see is what your ship will follow when the planet will be at its target. You have to use some imagination and take the planet from where it is, to put it where it will be during flyby. Only when you enter a planet's sphere of influence, you can see the path your ship will have after flyby. Orientation tips: Watch the path your ship will have around the planet. *If the ship, after the flyby, will exit the sphere of influence away from the Sun, the ship will turn to an orbit closer to the Sun. *If the ship, after the flyby, will exit the sphere of influence towards from the Sun, the ship will turn to an orbit further from the Sun. The same happens if the ship performs a Moon flyby, in relation with the Earth. In order to perform gravity assists, a ship has to do trajectory correction maneuvers and deep space maneuvers. See Also *Advanced Orbital Mechanics *Aerobrake *Deep Space Maneuvers *Oberth Effect *Trajectory Correction Maneuvers Category:Gameplay